The present invention relates to a technique for recovering a heat recoverable article.
A heat recoverable article is an article the dimensional configuration of which may be made substantially to change when subjected to heat treatment. Usually these articles recover, on heating, towards an original shape from which they have previously been deformed but the term heat-recoverable, as used herein, also refers to an article which, on heating, adopts a new configuration even if it has not been previously deformed.
In their most common form, such articles comprise a heat-shrinkable sleeve made from a polymeric material exhibiting the property of elastic or plastic memory as described, for example, in U.S. Pat. Nos. 2,027,962, 3,086,242 and 3,957,962. The disclosures of these patents are incorporated herein by reference. As is made clear in, for example, U.S. Pat. No. 2,027,962, the original dimensionally heat-stable form may be a transient form in a continuous process in which, for example, an extruded tube is expanded, whilst hot, to a dimensionally heat-unstable form but, in other applications, a preformed dimensionally heat-stable article is deformed to a dimensionally heat-unstable form in a separate stage.
In the production of heat-recoverable articles, the polymeric material may be cross-linked at any stage in the production of the article that will enhance the desired dimensional recoverability. One manner of producing a heat-recoverable article comprises shaping the polymeric material into the desired heat-stable form, subsequently cross-linking the polymeric material, heating the article to a temperature above the crystalline melt point, or, for amorphous material, the softening point, as the case may be, of the polymer, deforming the article and cooling the article whilst in the deformed state so that the deformed state of the article is retained. In use, since the deformed state of the article is heat-unstable, application of heat will cause the article to assume its original heat-stable shape. The present invention is concerned with a technique by which heat recovery may be induced.
Heat-recoverable articles find particular use in the encapsulation, insulation or mechanical protection of substrates such as electrical cables, optical fiber cables and other supply lines. The function of the recoverable article can be enhanced if it is used in conjunction with a sealant. Where the article is hollow, a sleeve or a boot for example, it may be coated on its inner surface with an adhesive in order to avoid any leak path between the article and the substrate to be protected. Such sealant is preferably heat-activatable, and may be a mastic which softens with heat, a hot-melt adhesive which melts, or an epoxy or other reactive system which cures. In each of these cases recovery of the article and activation of the adhesive can be performed by a single heating operation.
The most widely used means of heating is a butane or propane torch. A gentle flame is played over the recoverable article to cause recovery, and, if applicable, activation of an adhesive, without causing any damage to the article or the substrate over which it is being recovered. This method has the advantages that the same equipment can be used on most sizes of recoverable articles and for most recovery temperatures, and that the equipment is readily available; however, some skill is required if uniform recovery without damage is to be achieved and it is wasteful of energy since much is lost in convection and conduction. A more serious disadvantage is that naked flames are not permitted in hazardous areas, and a torch cannot therefore be used for effecting repairs there. Repairs on leaking gas pipes, unless proper ventilation is provided, are preferably carried out by methods which avoid the use of a naked flame.
One source of heat which avoids a naked flame is hot air. For smaller recoverable articles, such as electrical connectors marketed under the Raychem trade mark Solder Sleeve, a hot-air gun powered by electricity may be used. Such a technique has not, however, found wide use with larger articles.
A further heater that has been used in hazardous areas is the catalytic heater. A reasonably effective heat source is provided by clamping around the recoverable article a rigid hinged structure containing a chemical compositon which initiates an exothermic reaction when supplied with propane or other combustible gas. The device however is rather bulky and does not conform closely to the article to be recovered. Any one size of catalytic heater can be used only on a very small range of sizes of recoverable article.
Exothermic reactions have also been made use of, for example in U.S. Pat. Nos. 3,551,223, 3,924,603, 4,170,296 and 4,265,216.
In the first of these, a device is provided for heating a heat recoverable article, the device comprising a shield, one surface of which is close to or contacts the article, the other surface being covered with a material capable of liberating a predetermined amount of heat energy substantially uniformly around the surface of the recoverable article. The material is lit for example with a match, and then burns uniformly over the surface of the shield. This technique clearly cannot be used where naked flames are not permitted.
U.S. Pat. No. 3,924,603 discloses a method of shrinking a sleeve holding together two cables. A portable heat source containing for example a metal salt, or polyalcohol and an amine is conformed around the sleeve. An exothermic reaction between these chemicals generates the heat for recovery.
In U.S. Pat. No. 4,170,296 a heat recoverable article passes through a box having two compartments, into each of which has been placed one component of a two part exothermic reaction mixture. The partition separating the two compartments is removed allowing the two components exothermically to react.
A flexible container, again divided into two compartments containing reaction components, is disclosed in U.S. Pat. No. 4,265,216. This flexible container is placed around an article to be recovered, and the partition dividing the two compartments is removed or ruptured. A further feature is however introduced: the container itself has at least one face which is recoverable and therefore follows the change in shape or size of the article to be recovered. This system will, in general, only be useable once since the components will then have undergone an irreversible exothermic reaction and the container will have recovered.
We have now devised a way of inducing recovery which is applicable to a wide variety of sizes of recoverable articles, which does not require a naked flame and which involves equipment that is reuseable. This is done by using a liquid to transfer heat energy from the initial energy source to the recoverable article. Although the invention is of primary use in causing heat recovery, it can be used to cause heating for other purposes.